In recent years, a power supply device and a lamp fitting have been actively developed, which use, as a light source, a semiconductor light-emitting element such as a light-emitting diode (LED) in place of a halogen lamp and a discharge lamp. As performance such as light emission efficiency of the LED has been enhanced, these types of the power supply device and the lamp fitting have been expanded to be used not only for a room lamp and a rear combination lamp for a vehicle but also for a headlamp for the vehicle. A light source such as the halogen lamp and the discharge lamp (HID lamp), which has been heretofore used as a light source for the headlamp, is standardized, and a shape, characteristics, and the like thereof are determined. As opposed to this, under the present circumstances, the light source using the LED is not standardized, and at least for the present, a variety of the LEDs and combinations thereof will be set in response to specifications of the headlamp (Patent Literature 1, Patent Literature 2).
Moreover, in a DC-DC converter as a circuit for lighting such an LED load, there has been proposed a control method for turning on and off a switching element so that a current flowing through an inductance element that composes the DC-DC converter can flow in a critical mode (also referred to as a current boundary mode and the like) (Patent Literature 3). To drive the DC-DC converter in the current boundary mode is such a control method that has been examined and implemented in various ways for the conventional HID lamp. It is conceived that a loss can be reduced by using this control method also for the LED.
Incidentally, the semiconductor light source such as the LED is a low-impedance load having a predetermined forward voltage. Therefore, the load such as the semiconductor light source has characteristics that a large current ripple is prone to occur in the current flowing therethrough even if a voltage to be supplied thereto only has a small ripple component. This matter also results in that a current having a large ripple component flows through a wire connected to the load, also leading to an occurrence of unnecessary radiation noise. Accordingly, measures against noise are required.